I have added a category in the left column for ‘Lists of Commentaries’. So far, the only one is for Seneca’s Epistulae Morales, though Ovid’s Heroides will follow. This is not a complete bibliography, but a cross-reference of letters against commentators. I made it for my own use, since it is so hard to keep track of who comments on which letter, and post it now since others may find it useful. Similar lists would be very helpful for (e.g.) Pindar: if anyone has one I would be glad to link it or host it.
I have previously posted a one-page PDF version of the Seneca list, and that will be updated soon to reflect the latest information: I only recently learned of the commentaries on Epistles 22 and 23 by Laudizi and 93 and 99 by Op het Veld.
The list is not just a practical tool. It is interesting to see which letters have attracted multiple commentaries, and which (about a third) have attracted none. Editors have their own prejudices: for instance, Motto’s school commentary has a strong preference for the shortest letters, while Summers has a strong preference for those in the range 76-90, commenting on no fewer than 10 1/2 of the 15. If I have missed any, I hope someone will let me know.